Thousands of popular websites including Wikipedia, Reddit and Boing Boing have shut down for as long as 24 hours to protest congressional antipiracy bills they say amount to censorship.

This screen shot shows the home page of Google.com, which opposes the Stop… (Associated Press )

In the first strike of its kind, thousands of popular sites such as Wikipedia, Reddit and Boing Boing shut down for up to 24 hours Wednesday to protest a pair of federal antipiracy bills that they said amounted to censorship of the Internet.

The online grass-roots campaign is directed at the Stop Online Piracy Act and the Protect Intellectual Property Act, which aim to crack down on foreign websites that traffic in pirated movies, music and counterfeit goods.

To protest the bills before they go to a vote, pages on Wikipedia's English language encyclopedia site have gone dark and now feature a short note that tells visitors to "Imagine a World Without Free Knowledge."

The note went on to say: "Right now, the U.S. Congress is considering legislation that could fatally damage the free and open Internet. For 24 hours, to raise awareness, we are blacking out Wikipedia."

Visitors are given a link where they can learn more and then are urged to contact local legislators by plugging in a ZIP code.

Reddit's site asked users to "please take today as a day of focus and action." Its appeal will remain on the social news community's pages until 5 p.m. PST. Pages on group blogging site Boing Boing have been replaced with a memo claiming that the proposed legislation "would unmake the Web" and "certainly kill us forever."

The prospect of a day without the websites set off a frenzy in the hours leading up to the strike, which began late Tuesday night with parents urging their children to do their schoolwork early and tech-savvy users posting instructions for how to access cached Wikipedia pages during the blackout.

But many Wikipedia users were still caught unaware Wednesday, leading to reams of frantic rants — many filled with profanity — on sites such as microblogging hub Twitter.

Carter Bays, creator of the popular television show "How I Met Your Mother," tweeted a relatively calm message: "If the #HIMYM episode I'm writing is full of glaring factual errors that a quick peek at Wikipedia could have corrected, blame SOPA."

The shutdown of the other sites and the ensuing anxiety underscored the breadth and influence of the world's Internet companies, as well as Americans' dependence on them.

Web companies have broadened the debate on SOPA and PIPA, recasting it from one about piracy and digital copyright protection to one about Internet freedom. Calling the bills well intentioned but seriously flawed, they said SOPA in the House and PIPA in the Senate are threats to free speech that could stifle the Internet economy, drive up legal costs and lead to censorship or the shutdown of some websites.

The proposed legislation "creates a punishing Internet censorship regime and exports it to the rest of the world," Boing Boing said.

"Boing Boing could never coexist with a SOPA world: We could not ever link to another website unless we were sure that no links to anything that infringes copyright appeared on that site," the company said.

Erik Martin, general manager at Reddit, said the bills were "an existential threat to our company and the industry we work in."

"We try to be pretty agnostic when it comes to content, news, politics," Martin said. "This is the first time we've really stepped out and made a strong statement as a company. We feel we don't have a choice."

Calling the move "unprecedented," Wikipedia said that it believed the bills, if passed, "would be devastating to the free and open Web."

"In making this decision, Wikipedians will be criticized for seeming to abandon neutrality to take a political position. That's a real, legitimate issue. We want people to trust Wikipedia, not worry that it is trying to propagandize them," said Sue Gardner, executive director of the Wikimedia Foundation. "But although Wikipedia's articles are neutral, its existence is not."

But supporters of the legislation said the online campaign is misguided. In a statement Tuesday, House Judiciary Committee Chairman Lamar Smith (R-Texas), lead sponsor of SOPA, called the Wikipedia blackout a publicity stunt "that does a disservice to its users by promoting fear instead of facts."

"It is ironic that a website dedicated to providing information is spreading misinformation about the Stop Online Piracy Act," Smith said. "Perhaps during the blackout, Internet users can look elsewhere for an accurate definition of online piracy."

But the Internet sites aren't backing down.

Google Inc.was operating, but with its logo on its home page blacked out; it linked to a separate site highlighting its opposition to the bills. Craigslist blacked out its main page with a message beseeching users to contact Congress members, though visitors could still access the site's regional pages. Craigslist also featured an addendum telling "corporate paymasters" to "keep those clammy hands off the Internet."